If John Kerry's seat needs to be filled next year, it will be the fourth time Massachusetts voters have chosen Senate candidates in a six-year period. Actor-director Ben Affleck said Monday he won't run.

Sen. John Kerry D-Mass., speaks with actor Ben Affleck during a meeting with foreign relations members to discus the crisis in the Democratic Republic of Congo on Capitol Hill in Washington on Wednesday.

Jose Luis Magana/AP

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Massachusetts seems to have a thing about Senate races. The state is holding elections more often than usual for US Senate seats, and in ways that garner the national spotlight.

First, remember what's typical under the US Constitution: Each state has two Senate seats, with each coming up for election every six years, on a staggered schedule.

Then, think about this string of events: Massachusetts residents appear poised to cast ballots for the US Senate four times in a six-year period, or twice the typical pace. John Kerry (D) won a reelection vote in 2008. Then, after the passing of Sen. Edward Kennedy (D), the state held a special election in early 2010 that drew national attention with Scott Brown's surprise Republican victory.

Then came the epic battle, just concluded last month, in which Elizabeth Warren took that seat back for Democrats. Now, a few weeks later, with President Obama nominating Senator Kerry to be his next secretary of State, the stage is set for another high-profile special election in the new year.

Mr. Brown, a winner in 2010 and a loser in 2012, may be back as a Republican candidate in 2013.

He won't be running against Ben Affleck. The actor-director on Monday took his name off the list of possible Democratic contenders.

But whoever ends up running, assuming that Kerry is confirmed as secretary of State, the race is sure to be a magnet for media attention and money. It would be one of the few opportunities in 2013 for voters to have a say about who sits in Congress at a time of hot fiscal debates and narrow division of power between parties.

Back in 2010, Brown's upset win foreshadowed the conservative gains later that year that would tip the House of Representatives back into Republican control. This time around, the race could become, in part, an early verdict on the performance of both parties on things like tax rates, budget deficits, and how to keep programs like Medicare and Social Security financially healthy.

Whoever wins would have to turn around and start campaigning all over again pretty quickly, since the seat is up for a scheduled vote in 2014.

How did Massachusetts get into this flurry of Senate activity?

Yes, the answer involves the death of one senator and the appointment of another to a presidential cabinet post. But it's also about decisions made in the State House in Boston about how to handle such vacancies.

Some 36 states let the governor wield the power of appointment to fill a Senate vacancy – even if the unexpired term has years to run.

The other 14 states, including Massachusetts, have chosen to fill vacancies with special elections. Most of these states, in addition, provide for the governor to appoint an interim senator until the special election is held, according to the National Conference of State Legislatures.

In Massachusetts, Gov. Deval Patrick (D) has said he would follow a pattern he set in 2009 after the death of Mr. Kennedy, when he appointed an interim senator (Paul Kirk) who had pledged not to run in the special election. By adopting that course, Governor Patrick could avoid playing the controversial role of putting one person on a favored track in the Democratic primary.

Under state law, a special election must be held within 145 to 160 days after a vacancy occurs. If Kerry's appointment is confirmed by the Senate and the vacancy becomes official in January, that would put the vote around midyear.

(Prior to 2004, Massachusetts had the other system – where the governor would appoint somebody to fill a Senate vacancy for the remainder of the term. But Mitt Romney (R) was governor then, and some state lawmakers were concerned that if Kerry were elected president that November, Mr. Romney would appoint a Republican, meaning that Democrats would lose that Senate seat.)

One thing's for sure now: Despite the Bay State being predominantly liberal in its orientation, Democrats won't take this Senate seat for granted.

Brown hasn't declared his intentions yet on the Republican side, but he enjoys name recognition, personal popularity, and a reputation for political moderation and independence. There would also be a factor that helped him in his successful bid in 2010: a lower-turnout environment, when other key races won't be on the ballot. In contrast, last month voting turnout in Massachusetts was driven not just by whether they were enthusiastic about Ms. Warren or Brown, but also by the presidential race, House elections, and several ballot initiatives.

Still, if Brown does run, he wouldn't be a shoo-in. His high approval ratings among voters didn't push him over the top against Warren.

Democrats who might seek to replace Kerry include Reps. Edward Markey, Michael Capuano, and Stephen Lynch. Those three are all from the greater Boston area, all have solid credentials as politicians, and all lack the celebrity appeal of Mr. Affleck – judging by the number of Twitter followers.

"I love Massachusetts and our political process, but I am not running for office," Affleck said in a Monday Facebook post that doused a flurry of speculation that he might run. "Right now it's a privilege to spend my time working with Eastern Congo Initiative (ECI), supporting our veterans, drawing attention to the great many who go hungry in the U.S. everyday and using filmmaking to entertain and foster discussion about issues like our relationship to Iran."

Affleck, who grew up in Massachusetts, added, "We are about to get a great Secretary of State and there are some phenomenal candidates in Massachusetts for his Senate seat. I look forward to an amazing campaign." Then he signed off with, "Happy Holidays to All."